An “EarLens™” is a contact transducer assembly for a hearing aid. Examples of EarLenses are described by Perkins and Shennib in U.S. Pat. No. 5,259,032 (“Perkins”), incorporated herein by reference. An EarLens is a biocompatible, removable contact transducer that is supported on the tympanic membrane of a wearer without the use of adhesives or mechanical attachments. The name “EarLens” was chosen because the installation and removal of the contact transducer assembly is reminiscent of the method for insertion and removal of contact lenses for the eyes. Furthermore, surface wetting agents may be used with the EarLens in analogy to the use of wetting solutions for contact lenses for the eyes.
Hearing aids based on EarLenses or other contact transducers offer potential advantages over conventional systems. A conventional hearing aid uses an acoustic transducer to generate amplified sound waves in air that are received by the tympanic membrane (eardrum) of a wearer. In contrast, a contact transducer is one that imparts vibrations directly to the tympanic membrane. Simply put, a contact transducer touches the eardrum while an acoustic transducer is a very small loudspeaker that is located near, but not touching, the eardrum. (FIG. 1 is a diagram of the human auditory system including the ear canal, tympanic membrane, ossicles, and the cochlea.)
Vibrations of the tympanic membrane are perceived as sound regardless of whether the vibrations are stimulated by sound waves in air or by a contact transducer. Contact transducers offer better sound quality than acoustic transducers and they reduce feedback problems. However, the development of hearing aids using contact transducers has taken many years. The Perkins EarLens eliminates problems associated with adhesives that were previously used to attach transducers to the eardrum and it does not require surgery.
Currently EarLens contact transducers are custom fitted for each wearer by making a negative impression of the wearer's eardrum. A positive mold is then created and a biocompatible material is cast or molded from the positive impression. EarLenses also use a surface wetting agent to enhance sufficient, but weak, adhesion to the tympanic membrane while allowing the tympanic membrane epithelium below to naturally migrate. Mineral oil is often used as a surface wetting agent and may be periodically sprayed into the ear canal after placement of an EarLens device.